In order to allow proteins bearing a principal role in the biological life functions to orderly and properly exhibit their functions in vivo, post-translation modifications including glycosylation play an extremely important role. With regard to the post-translation modifications, the following findings were gradually made in recent years. The majority of the proteins in a living body are modified with sugar chains, and those sugar chains attached to the proteins play important roles in various aspects of biological phenomena, including protein stability, binding with hormones, binding with toxins, viral infection, mycoplasma infection, bacterial infection, protozoan infestation, fertilization, development and differentiation, cancer cell metastasis, apoptosis and the like. Even when proteins have the same amino acid sequence and the same name, such proteins are modified with a wide variety of sugar chains and, depending on the condition of the protein-producing cells, the structures of the sugar chains are variable and the proteins thus have different roles in vivo.
The relationships between such changes in sugar chains and diseases have also been gradually elucidated. For example, Patent Document 1 describes as follows with regard to prostate-specific antigens (hereinafter, referred to as “PSAs”) which indicate that a subject has a prostate disease. That is, it is described that, as compared to blood samples originated from prostatic hyperplasia patients, those blood samples originated from prostate cancer patients contain a greater amount of a prostate-specific antigen having a specific sugar residue, i.e., an N-acetyl-D-galactosamine β1-4 N-acetylglucosamine (hereinafter, referred to as “LacdiNAc”) residue (this prostate-specific antigen is hereinafter referred to as “LacdiNAc-PSA”) and/or a fucose-α(1,2)-galactose β1,4 N-acetylglucosamine residue, in its sugar chain. This means that the onset of prostate cancer changes the sugar chains of PSAs and this leads to an increase in the amount of PSAs having the above-described specific sugar residue(s), as a result of which a high concentration of PSAs having the specific sugar residue (s) is observed in blood samples of prostate cancer patients. On the other hand, since the onset of prostatic hyperplasia does not cause such a change in the sugar chain, prostatic hyperplasia patients are not observed with a change in the concentration of PSAs having the specific sugar residue(s). On the basis of this, Patent Document 1 discloses a method of distinguishing prostate cancer by fraction measurement of sugar chains, which method is capable of distinguishing a prostate cancer patient from a prostatic hyperplasia patient by measuring, in their blood samples, the concentration of PSAs having the above-described specific sugar residue(s). In addition, for example, a method of identifying liver cancer by fraction measurement of α-fetoprotein (AFP) sugar chains and a method of identifying adenocarcinoma by fraction measurement of carcinoembryonic antigen (CEA) sugar chains have also been proposed.
Further, mucin-1 (hereinafter, referred to as “MUC1”), which is one of mucins that are high-molecular-weight glycoproteins, is also known as a tumor-associated antigen. MUC1 is widely expressed in the epithelial cells of normal glands, and it is known that the expression of MUC1 is drastically increased when the cells are malignant, for example, in breast cancer, ovarian cancer, lung cancer, pancreas cancer and bladder cancer; and that the glycosylation pattern changes in breast cancer and the like. Patent Documents 2 and 3 disclose an aptamer ligand and an anti-MUC1 antibody, both of which can be used for the quantification of such MUC1, respectively. Moreover, Patent Document 4 describes that MUC1 and the like can be used as biomarkers of prostate cancer and colorectal cancer.
For specific detection of a glycoprotein containing a specific sugar residue in its sugar chain, proteins called lectin that are capable of specifically recognizing and binding to such sugar residue are widely utilized. This is because it is very difficult to prepare an antibody whose epitope is a sugar chain, particularly an antibody whose epitope is a specific sugar residue, and such an antibody is thus hardly available. Lectins not only are inexpensive and available in a large amount but also have excellent protein stability and can thus be stored over a long time.
For example, Wisteria floribunda lectin (Wisteria floribunda agglutinin: hereinafter, referred to as “WFA”) is known to have N-acetylgalactosamine as its primary binding sugar residue. Patent Document 1 discloses a method in which WFA having such a property is bound to a carrier and loaded to a column and a PSA having a LacdiNAc residue in a side chain of an asparagine-linked sugar chain is subsequently fractionated and quantified by ELISA or the like. In addition, Patent Document 5 discloses a method in which a solid-phase anti-PSA antibody and a fluorescently labeled WFA are allowed to forma sandwich complex with a PSA having a LacdiNAc residue in a side chain of its sugar chain and this PSA having the specific sugar residue is then quantified by SPFS (Surface Plasmon-field enhanced Fluorescence Spectroscopy).